Two days before you plan to serve the pork, combine the buttermilk, onion, salt, 3/4 cup maple syrup, and sage or other herbs in a large resealable plastic bag. Squish the bag with your hands to dissolve the salt and combine the syrup and buttermilk. Add the pork loin and seal the bag, removing as much air as possible. Refrigerate for two days, turning the bag occasionally.

Remove the bag from the refrigerator three hours before you plan to start cooking the pork to allow the meat to come to room temperature.

Preheat the oven to 300°. Heat the bacon fat in a large ovenproof skillet and swirl it to coat the bottom of the pan, then pour off the excess.

Drain the marinade from the meat, pat the meat dry with paper towels, then sprinkle it liberally with freshly ground pepper. Brown the meat on all sides in the skillet over moderately high heat, then put the meat, uncovered, in the oven.

After 45 minutes, combine the remaining 1/2 cup of maple syrup and the mustard. Brush the meat with some of the mixture. Repeat every 15 minutes until the mixture is used up, then continue to baste with the pan juices.

Roast until a thermometer inserted halfway into the thickest part of the meat registers 150 degrees. Total cooking time will be between one-and-a-half and two hours. Let the roast stand for 15 minutes, then slice thinly and serve with the pan juices. Serves six to eight. Leftovers make great sandwiches.